1. Field of the Invention
This invention relates to a process for conveying gas by means of a liquid ring pump. The invention also relates to an apparatus for carrying out said process.
2. History of the Prior Art
Liquid ring pumps are often used for conveying gas, particularly chemically reactive gases and vapors. Such pumps are self-aspirating pumps which are normally displacement-type pumps when they are used for conveying gas. A bucket wheel eccentrically mounted within a pump housing with one side of it coming quite close to the inner wall of the pump housing centrifugally forces the functional fluid in the pump housing to the inner wall of the pump housing, resulting in the formation of a substantially concentric fluid ring whereby the single cells of the bucket wheel are effectively sealed off with respect to each other in a radial direction. Hollow spaces are formed in the single sectors of the bucket wheel. Those hollow spaces create suction and pressure action, respectively, caused by the rotation of the bucket wheel. The intake for the gas is connected with the space under suction whereas the outlet of the pump is connected with the space under pressure.
Liquid ring pumps are typically employed both as vacuum pumps and as compressors, and may be run at high rotational speed.
A principal difficulty in operating such ring pumps is that the gas which is to be conveyed thereby is brought into immediate contact with the functional fluid within the pump, so that the gas or individual components of the gas can be dissolved in substantial amounts in the functional fluid, particularly when the fluid and the gas are under pressure. If this is the case, the liquid ring pump may be separately cooled to reduce gas absorption, i.e., instead of a single functional fluid the system employs a working liquid and a separate cooling liquid. For this purpose, the working liquid is circulated through a surface condenser which is cooled by the cooling liquid, e.g., water.
Under the above mentioned circumstances, the working fluid dissolves the gas to be conveyed until the concentration of gas absorbed reaches a saturation limit which is dependent on the prevailing process parameters. When the saturation limit is reached, no further gas is absorbed by the working liquid.
Another difficulty in connection with the use of liquid ring pumps for conveying gas is that the gas to be conveyed often contains water vapor which can condense in the liquid ring pump during its compression. The amount of condensed water vapor is a function of the temperature of the moist gas to be conveyed at the intake of the pump. Vapors other than water in the gas to be conveyed may also show a similar behavior.
In case a condensate is formed in the liquid ring pump the condensate must be removed continuously from the working liquid. The condensate so removed may contain the conveyed gas or its components, respectively, dissolved in amounts corresponding to saturation conditions. It does not matter whether the working liquid and the condensate are identical compounds, e.g., water, or whether the working liquid and condensate are chemically different compounds which are in partial equilibrium.
The loss of gas caused by the necessary removal of the condensate formed in the surface condenser while the pump is working often is relatively small and, in most cases, tolerable. Thus, if the gas which is to be conveyed is substantially soluble in the working liquid, surface cooling of the working liquid in the liquid ring pump is the method of choice to limit such gas loss with the condensate. However, a disadvantage of this method is the requirement of heat exchange between two liquids separated by a surface condenser, which involves high operating and investment costs.
A further disadvantage of surface cooling of the working liquid is that the amount of gas sucked by the pump is dependent on the temperature of the cooling liquid. The higher the temperature of the cooling liquid is, the lower is the suction power of the liquid ring pump. When a given suction power of the pump must be attained by use of a cooling liquid with a maximum operating temperature which is ordinarily not very low, the liquid ring pump must be large in order to yield the necessary suction power at this maximum temperature of the cooling liquid. In this context, it is important to note that the temperature of the working liquid is about 5.degree. to 10.degree. C. higher when a surface condenser is used than it would be if the pump were cooled directly without the use of a surface condenser by the same amount of cooling liquid. This temperature increase in 5.degree. to 10.degree. C. can correspond to a decrease of the suction power of the liquid ring pump up to almost 25%. Despite such limitations, liquid ring pumps with separate working and cooling liquids have succeeded generally because of the balance of factors involved. An example to illustrate this is presented below:
A spinning bath as used in the manufacturing of rayon staple, is to be degassed in a container under low pressure. In this process, primarily hydrogen sulfide and carbon disulfide are liberated. The liberated gases are sucked off by means of a liquid ring pump with a suction power of 700 to 800 m.sup.3 /h at a temperature of about 30.degree. C. and a pressure of 120 mbar. Provided that direct cooling is used, 2 m.sup.3 /h cooling water at a temperature of 20.degree. C. would be required in order to cool the liquid ring pump under the above-mentioned conditions. Under those conditions, a stream of cooling water, for direct cooling of the ring pump, when saturated with the gases would dissolve about 4 kg/h carbon disulfide and about 6 kg/h hydrogen sulfide. It is apparent that such a loss of gas cannot be accepted not only in view of the loss of material itself but also due to the contamination of the cooling water thereby.
Despite the mentioned disadvantages in connection with separate circulating of the working liquid and the cooling liquid, the balance of factors speaks for the use of the latter process. Only 30 to 40 kg/h water vapour would condense in the liquid ring pump and have to be removed from the working liquid if the process were run at the same parameters and had the same cooling power as provided by a surface condenser. The mentioned amount of water vapor is a typical value depending on the water content of the gas, the temperature and pressure to which the gas is compressed. 30 to 40 kg/h condensate saturated with carbon disulfide and hydrogen sulfide can dissolve about 60 to 80 g/h carbon disulfide and 90 to 120 g/h hydrogen sulfide. In other words, if one would directly cool the working liquid, in a process for conveying gas which is readily soluble in the working liquid, by means of a liquid ring pump, there is a loss of gas 60 times higher than in a process and apparatus employing a surface condenser that is switched between the working liquid and the cooling liquid. The higher operation and investment costs for the relatively large liquid ring pump and the additional surface condenser, however, can be accepted in view of the above mentioned losses in a process for conveying gas by means of a liquid ring pump which is immediately cooled.